1. Field of the Invention
The present invention relates generally to solar cells and, more particularly, to a back junction photovoltaic solar cell.
2. The Prior Art
Conventional silicon photovoltaic solar cells, intended for operation over an input intensity range of 80 mW/cm.sup.2 to 100 W/cm.sup.2 (approximately 1 sun to 1000 suns), have a conversion efficiency in the range of 15 to 20 percent. Such solar cells, whether they are intended for high or low intensity operation, generally comprise a standard p-n junction formed at the front surface of silicon substrate. A metal grid contact is formed upon the front surface to collect photo-generated current and a full-area contact is applied to the rear surface.
The conventional solar cell has several limitations that are a consequence of the cell design. One important limitation arises at high input intensity because large amounts of current must be conducted from the front surface of the device, through a metal grid contact, to wires or leads that connect the device to an external circuit. Power loss owing to series resistance increases as the square of the current, thus increasing as intensity increases. The metal grid cannot be made arbitrarily large without reducing the photo-generated current, since the grid blocks light from entering the cell. Owing to numerous constraints on grid design that are necessary for both minimizing series resistance and maximizing the amount of light that enters the device, other aspects of cell design, such as junction optimization, cannot also be simultaneously optimized.
An unconventional approach to the reduction of series resistance has been termed the etched multi-vertical-junction (EMVJ) cell. The principal feature of the EMVJ cell is the use of deep channels that extend into the bulk of the substrate. Since the p-n junction and metallization are formed over the front surface of the cell and within these channels, the amount of grid metal is increased without an increase in shadow loss, thus reducing series resistance and resulting in a net gain in efficiency, particularly during operation at very high intensity, during which series resistance is the most important loss mechanism. An alternate approach comprises formation of V-groove across the front surface. Grid metallization placed properly within the grooves has negligible shadow loss, because light blocked by the grid is reflected onto a non-obscured region of the front surface. Neither the V-groove design nor the EMVJ design permit optimization or patterning of the p-n junction. Whereas the conventional cells are limited by series resistance, the foregoing alternatives are limited by low open circuit voltage owing to non-optimal junction design.
A third alternative is the interdigitated back contact (IBC) cell design. The IBC design comprises a back p-n junction cell in which both p and n contacts are in the form of interdigitated grids on the rear surface. The p-n junction is patterned on the rear surface and is in this way similar to the invention disclosed here. However, the IBC design does not separate the grid pattern and the junction pattern and thus the two are constrained in the same way that a front junction grating or pocket cell design is constrained. Thus, the IBC design does not decouple grid and junction designs. The invention disclosed herein overcomes both problems by new device design.